The present invention relates to protection of electrical systems and more particularly relates to a method of measurement, analysis and discrimination of signals for the purpose of determining leakage and/or fault currents in electrical devices supplied by such systems. The invention also relates to the identification of frequency components of currents in electrical systems. The invention further relates to methods of detection of high frequency current earth faults and leakages in particular though not exclusively in I-T (isolated terra and/or earth fault limited) supply networks and particularly supply networks in hazardous areas such as but not limited to mines. The invention also relates to a method of improving the reliability of relays in determining earth leakage faults and particularly when the relays are operating in association with switching power electronics, including but not limited to variable speed drives. The invention further relates to a method for interpreting wide bandwidth measurement signals to flag potential hazards that may result from operation of switching electronics rather than rejecting such signals as noise to simplify analysis. The method includes real-time analysis of signals from multiple transducers or measurement points to provide fault diagnostic and location information in I-T supply networks.